Synthetic papers formed of fully aromatic polyamide (aramid) is known as a heat resistant paper having a good insulating property for use in a transformer and motor. Among them, a heat-resistant paper made from a mixture of an aramid pulp and aramid short fibers is well-known as a heat-resistant paper having a proper heat resistance and flexibility, and an excellent insulating property and dimension stability.
Polyimide is also well-known as a resin having an excellent heat resistance. In particular, aromatic polyimide is processed into various shapes and used since it has a very high heat resistance and excellent mechanical property. Non-woven fabrics and paper containing the short fibers of polyimide are also developed. For example, Patent document 1 discloses polyimide fibers obtained by spinning a thermoplastic polyimide resin and a polyimide paper containing the polyimide fibers. However, since fully aromatic polyimide has not occasionally a melting point depending on the structure thereof and may not be dissolved in a solvent, it may be hard to be spun. Therefore, it is proposed to produce short fibers using an acid solution composition containing polyamide that is a precursor of the aromatic polyimide as disclosed in Patent document 2. In general, in order to employ short fibers to produce non-woven fabrics and paper, it is important that the short fibers are interlocked each other and there preferably is a structure having a large number of branch. Therefore, the method is employed which comprises applying a shearing force on the fibers to form a pulp-like structure at spinning or after spinning.
Further, patent document 3 discloses a polyimide foam which is formed of polyimide having a glass transition temperature higher than 300° C., and has an expansion ratio of at least 20 times, and a method of producing the same. Patent document 4 discloses a foamed polyimide molded product manufactured by a method including crushing a polyimide foam, mixing the crushed polyimide foam with a heat resistant binder, introducing the mixture into a prescribed mold, pressing the mixture until a density thereof reaches to a certain value, and heating the pressed mixture.